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| United States Patent |
5,037,794
|
|
Magistro
|
August 6, 1991
|
Attrition resistant catalyst support
Abstract
Alpha-alumina particles having a combination of low attrition, thermal
stability and fluidizability are disclosed. The alpha-alumina particles
are characterized by being devoid of crystalline grain boundaries. The
particles are useful as support or carriers for a wide variety of
catalysts, especially as catalyst carriers in high temperature catalystic
processes.
| Inventors:
|
Magistro; Angelo J. (Brecksville, OH)
|
| Assignee:
|
The B. F. Goodrich Company (Akron, OH)
|
| Appl. No.:
|
405909 |
| Filed:
|
September 12, 1989 |
| Current U.S. Class: |
502/355; 423/625; 423/628; 502/415; 502/439 |
| Intern'l Class: |
B01J 021/04 |
| Field of Search: |
502/355,439,415
|
References Cited
U.S. Patent Documents
| 3898184 | Aug., 1975 | Hara et al. | 502/313.
|
| 4019978 | Apr., 1977 | Miller et al. | 502/211.
|
| 4111853 | Sep., 1978 | Shultz et al. | 252/536.
|
| 4170570 | Oct., 1979 | Zagata et al. | 502/211.
|
| 4280929 | Jul., 1981 | Shaw et al. | 502/215.
|
| 4379134 | Apr., 1983 | Weber et al. | 423/628.
|
| 4453006 | Jun., 1984 | Shaw et al. | 502/200.
|
| 4770869 | Sep., 1988 | Misra et al. | 502/415.
|
Primary Examiner: McFarlane; Anthony
Attorney, Agent or Firm: Dunlap; Thoburn T.
Claims
What is claimed is:
1. A catalyst support consisting of an attrition resistant fluidizable
alpha-alumina particle wherein said particle is substantially devoid of
crystalline grain boundaries, cracks and fractures and having an attrition
number not exceeding 30 as measured by the Roller attrition test.
2. The catalyst support of claim 1 wherein the alpha-alumina is spheroidal.
3. The catalyst support of claim 1 having an attrition number not exceeding
15.
4. The catalyst support of claim 1 having an attrition number not exceeding
10.
5. The catalyst support of claim 1 wherein said alpha-alumina is thermally
stable between about 500.degree. C. and 1000.degree. C.
6. A catalyst support consisting of a fluidizable, attrition resistant
alpha-alumina particle wherein said particle is substantially devoid of
crystalline grain boundaries, cracks and fractures and having an attrition
number not exceeding 5 as determined by the Roller attrition test.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to attrition resistant alumina catalyst supports.
More particularly, this invention relates to fluidizable alpha-alumina
catalyst supports having improved attrition and high temperature
resistance. The alpha-alumina supports of the present invention can be
doped or impregnated with appropriate active metal(s) to obtain desired
catalytic properties.
2. Background
Those skilled in the art have long recognized the advantages of fluid-bed
catalytic processes over fixed-bed catalytic processes. Such advantages
include improvement of temperature control and heat transfer, resulting in
greater reactor efficiencies. The activity, efficiency, stability, and
durability of a catalyst in a fluidized-bed catalytic reaction depend, to
a large degree, upon the structural and physical properties of the
catalyst support material. A problem, however, with the use of certain
support materials is attrition of the support particles by abrasion of the
surface of the support particle or fracture of the support particle
itself. Excessive particle attrition is caused, for example, by particle
to particle contact, abrasion with bed walls and bed internals, as well as
distributor jet impingement and abrasion in circulation conduits leading
to and from the reactor bed. High particle attrition contributes to
product contamination, catalyst loss, plugging of downstream equipment,
high filtration costs, and unstable fluidization behavior such as
channeling, slugging or increased entrainment of reactants. The
deleterious effects of fluidized-bed operations are exacerbated by high
temperature conditions.
It has been well-known that aluminum oxide (Al.sub.2 O.sub.3) is an
excellent support material for catalysts in a wide range of chemical
reactions. Various forms of aluminum oxide (hereinafter referred to as
alumina) occur in nature and many have been produced synthetically. Among
the conventional catalyst supports for use in catalytic processes, most
have been produced from gamma-alumina which is generally characterized by
having high surface area, low bulk density, and high mechanical strength.
Under high temperature conditions, however, gamma-alumina undergoes
changes through various crystalline phases (e.g., delta, eta, theta,
kappa, chi, and rho), ultimately transforming into alpha-alumina.
Alpha-alumina, the final product of aluminum oxide thermal transformation,
is chemically and thermally stable. Because of its thermal and chemical
stability, it would be highly desirable to utilize alpha-alumina as a
catalyst support in high temperature catalytic processes. However, the
aforementioned crystalline transformation is accompanied by a large
reduction in surface area with the formation of irregularly shaped,
brittle particles that are highly subject to attrition, and an almost
complete loss of mechanical strength. Consequently, alpha-alumina has
found little use in high temperature catalytic processes, particularly as
a carrier or support material for fluid-bed catalysts.
In the past, attempts have been made to produce alpha-alumina with
sufficient strength and attrition resistance in order to take advantage of
its inherent thermal and chemical stability properties. It is known that
the attrition resistance of a support such as a fluid-type catalyst
support can be increased by incorporating binders into and onto the
catalyst support matrix. The use of binders, however, introduces
additional entities into and onto the support that may have their own
reactivities, resulting in competing side reactions. A further
disadvantage with binders is that they can decrease the surface area,
increase the bulk density, and decrease the pore volume of a catalyst
support. In addition, most binders will not have sufficient thermal
stability to be useful in many high temperature catalytic processes.
The commercial utility of catalyst compositions in reactions which involve
conditions of high stress (such as high temperatures and/or pressure,
especially under fluidized-bed conditions) require support or carrier
materials that are highly resistant to abrasion and attrition. Catalyst
researchers continue to look for high efficiency catalysts and supports of
increased stability, physical strength, and attrition resistance, and that
are useful in reactions involving conditions of high stress. Thus, the
ability to employ attrition resistant catalysts supported on alpha-alumina
in fluidized-beds, particularly, under high temperature conditions without
the use of binders would be highly desirable.
SUMMARY OF THE INVENTION
Accordingly, it is a basic objective of the present invention to obviate
the deficiencies of conventional alpha-alumina.
In particular, it is an objective of the present invention to provide a
thermally stable and attrition resistant catalyst support material
suitable for fluid-bed catalytic processes.
It is a further objective of the present invention to provide a new and
novel alpha-alumina support which is suitable as a support for
catalytically active components.
The above and other objectives of the invention are accomplished by
providing a catalyst support comprising an inert substrate of
alpha-alumina, wherein the alpha-alumina ultimate articles are devoid or
substantially devoid of any fractures, cracks or crystalline grain
boundaries.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a diagramatic view of a prior art alpha-alumina primary
particle aggregate.
FIG. 2 is a cross-sectional view taken on line 2--2 of FIG. 1.
FIG. 3 represents a diagramatic view of a prior art alpha-alumina Primary
particle agglomerate.
FIG. 4 is a cross-sectional view taken on line 4--4 of FIG. 3.
FIG. 5 represents a diagramatic view of the alpha-alumina primary particle
of the present invention.
FIG. 6 is a cross-sectional view taken on line 6--6 of FIG. 5.
FIG. 7 is a photomicrograph of alumina trihydrate primary particle
aggregates at 1000.times..
FIG. 8 is a photomicrograph at 500.times. of the Ketjen gamma-alumina
precursors of the present invention.
FIG. 9 is a photomicrograph of alpha-alumina at 1000.times. prepared from
alumina hydrate.
FIG. 10 is a photomicrograph of the attrition resistant alpha-alumina
support particles of the present invention at 500.times..
FIG. 11 is a side elevation view of the Roller apparatus utilized to
conduct the attrition tests described.
DETAILED DESCRIPTION OF THE INVENTION
Upon examination under the electron microscope, the present inventor has
found that conventional gamma-alumina catalyst carriers are composed of a
plurality of irregularly shaped ultimate particles closely and rigidly
grouped or clustered together (e.g., aggregated or agglomerated). These
aggregated or agglomerated ultimate particles form what is herein referred
to as the primary particles. FIGS. 1 and 3 represent prior art
alpha-alumina primary particles 4 and 7 which are aggregations or
agglomerations of ultimate particles 5 and 8, respectively. The primary
particles are characterized by a myriad of channels, voids, fractures and
cracks that define the boundaries between adjacent ultimate particles. The
ultimate particles also contain grain boundaries, boundary lines
separating regions of the particles with different crystal structure
orientations. Although random in nature, these grain boundary lines are
substantially, uniformly distributed throughout a primary particle. FIGS.
2 and 4 are cross-sectional representations of the prior art primary
particles depicting crystalline grain boundaries 6 and 9.
When exposed to high temperatures, alumina sequentially transforms from an
amorphous or transitionally crystallized state (e.g., gamma, delta, eta,
etc.) to a highly crystallized state (e.g., alpha). During this
transformation the alumina ultimate particles become more crystalline
(accompanied by a reduction in surface area) and the crystalline grain
boundaries between adjacent ultimate particles become more stressed,
resulting in loss of adhesive forces between the ultimate particles. This
phenomenon significantly attributes to attrition. Any subsequent thermal
and/or mechanical shock delivered to the primary particles leads to
fracture along these crystalline grain boundaries resulting in attrition
of the ultimate particle.
When viewed under the electron microscope, it is readily seen that the
alpha-alumina of the present invention mainly exists of non-aggregated or
non-agglomerated particles that are substantially free of crystalline
boundaries. In other words, the primary particles are themselves the
ultimate particles. FIGS. 5 and 6 illustrate the morphology of a primary
particle of the present invention. Since the particles are devoid of
crystalline grain boundaries, they are resistant to attrition. The present
inventor has discovered that the attrition problem with conventional
alpha-alumina was found to be related to the starting structure
(morphology) of the gamma or other precursor alumina utilized in the
manufacture of the alpha-alumina. These precursor aluminas were found to
contain crystalline grain boundaries as previously described. FIG. 7, for
example, is a SEM photograph (scanning electron microscopy) of precursor
alumina trihydrate wherein the crystalline boundary lines between ultimate
particles are clearly evident. In sharp contrast, the precursor aluminas
to the attrition resistant alpha-alumina of the present invention contain
no crystalline grain boundaries as shown in FIG. 8. In order to have a low
attrition alpha-alumina catalyst support, the starting (precursor) alumina
particles should be devoid or substantially devoid of any crystalline
grain boundaries, fractures or cracks and should not consist of an
aggregation or agglomeration of ultimate particles.
Attrition resistance also depends on the physical form of the particles.
Spheroidal particles with smooth surfaces will have lower attrition losses
than particles with irregular shapes and rough edges. The term spheroidal
also is meant to include spherical, eliptical, oblong, globular, and the
like so long as there are no irregular or sharp edges that are prone to
attrit during handling or fluidization.
A satisfactory alumina starting material for the attrition resistant
alpha-alumina support of the present invention is gamma-alumina which is
commercially available from Ketjen, a subsidiary of Akzo Chemical BV,
Amersfoort, the Netherlands, under grade designations E and ES. These
particular grade designations of gamma-alumina are characterized by having
no crystalline grain boundaries when viewed under the electron microscope
and exist as a collection of unattached ultimate particles (as readily
seen in FIG. 8). It also should be realized that within the scope of the
present invention, any alumina precursor such as, for example, alumina
trihydrate and other gamma-alumina or transition phase alumina (e.g.,
delta, eta, theta, kappa, chi, and rho) may be utilized as precursor
materials so long as the foregoing conditions are met.
Most thermally induced solid state phase transformations in a
polycrystalline material such as alumina occur without retention of
particle morphology. FIG. 9 shows a photomicrograph of alpha-alumina
primary particles prepared from alumina trihydrate precursor pictured in
FIG. 7. Tests confirm that the alpha-phase is significantly more attrition
prone than its aluminum hydrate precursor (see Table II). The original
particles become weaker and/or fracture as they undergo transformation to
the new crystalline phase. Subsequent crystal grain growth, dependent upon
the reaction conditions, can further alter the shape and size of the
product particles. Surprisingly, SEM has shown that the morphology of the
starting or precursor alumina particles of the present invention is
maintained on conversion to the alpha-alumina phase. Consequently, the
resulting alpha-alumina particles are highly attrition resistant and
fluidizable. FIG. 10 is a SEM photomicrograph of the alpha-alumina
catalyst support particles of the present invention calcined at
1250.degree. C. Even at high conversion temperatures, the primary
particles are devoid or substantially devoid of cracks, fractures, and/or
grain boundaries.
The alpha-alumina catalyst support of the present invention is prepared
from the Ketjen precursors set forth above (or other aluminas,
gamma-aluminas or transitional aluminas that meet the specific requisites)
by any convenient method known in the art for preparing alpha-alumina from
gamma or other alumina precursors. In order to convert the precursor
alumina to the alpha-phase, the precursor material is heated to at least
about 1150.degree. C. Temperatures lower than about 1150.degree. C. do not
result in substantial conversion of the precursor alumina to the
alpha-alumina phase. While temperatures to about 1700.degree. C. or higher
may be employed, it should be noted that the alpha-alumina obtained at
higher temperatures have lower surface areas. The conversion is preferably
carried out between about 1150.degree. C. and 1300.degree. C., and most
preferably at about 1250.degree. C. A calcination time of about 4 to 24
hours is required to obtain substantially complete conversion to
alpha-alumina, although shorter or longer times (contingent upon
temperature) may be employed without detriment to the support. The
calcination may be effected in any calcination apparatus known in the art.
Non-limiting examples include ovens, muffle furnaces or tunnel furnaces
containing fixed beds or moving beds, rotary kilns, and the like.
The surface area of the resulting alpha-alumina is between about 0.1 to 14
m.sup.2 /g, preferably from about 3 to 10 m.sup.2 /g, and most preferably
from about 5 to 7 m.sup.2 /g. As noted above, the desired surface area can
be arrived at by controlling the calcining conditions (e.g., time and
temperature).
A special advantage of the fluidizable, attrition resistant, alpha-alumina
catalyst supports of the present invention is that the surface areas of
the support particles are relatively stable to changes in temperature and
chemical environment. This is advantageous in high temperature catalytic
processes in eliminating fluctuating reaction efficiencies due to changes
in the surface area of the support material. Accordingly, the
alpha-alumina catalyst supports of the present invention are particularly
useful in a variety of high temperature catalytic processes (e.g., between
about 500.degree. C. to about 1000.degree. C.), but can also be utilized
at lower temperatures where a high attrition support might be useful.
The alpha-alumina is suitable as a support or carrier for catalytic
materials, particularly a metal component, or metal components, as
employed in the manufacture of catalyst for chemical processing, crude oil
refining, and emission control.
The catalytic metal(s), usually in the form of salts, can be deposited,
incorporated in, or intimately associated with the alpha-alumina support
of the present invention. The simplest method involves intimately mixing
the metal salts with the attrition resistant alpha-alumina support
material of the present invention. The amount of metal salts utilized, of
course, will depend upon the specific catalytic reaction and the desired
rate of reaction and selectivity. The metal salts and alpha-alumina
support are wet mixed by slurrying the mixture in a suitable wetting
agent, for example, water or an organic compound such as methanol,
ethanol, and the like. The slurried mixture is then dried at a temperature
and for a time sufficient to substantially remove the excess wetting
agent. Generally, heating at a temperature of about 100.degree. C. to
about 250.degree. C., for 1 to 16 hours is sufficient. It will be
recognized, however, that the actual time and temperature depend upon the
particular wetting agent employed, the quantity of material and the like.
The supported catalyst can, if desired, be heated in an active or inert
atmosphere or calcined to achieve infusion or sintering of the active
metal (as the element, oxide or other combined form) into and onto the
support.
In an alternative method of preparation, a suitable precursor alumina (for
example, alumina trihydrate, gamma or any of the transitional aluminas
that meet the requisites set forth above) can be deposited with,
incorporated with, or intimately associated with the catalytic metal salts
as discussed previously, and dried, followed by calcination. It should be
noted, however, that the calcination conditions should be such so as to
convert the precursor aluminas into the alpha-form. The catalyst is dried
or calcined under non-agglomerating conditions to prevent aggregation of
the catalyst particles.
The alpha-alumina catalyst support of the present invention exhibits an
attrition index of less than about 30, more preferably less than about 15,
even more preferably less than about 10 and most preferably less than
about 5. The term "attrition index" as employed herein refers to percent
attrition as determined by the Roller attrition test. In this test, which
is conducted in a Roller apparatus and described in detail below, a
weighed sample of catalyst support material is subjected to an air jet
formed by passing humidified air at 21 liters/min. through a 0.07 in.
nozzle for one hour (initial phase). Initial phase fines are removed as
formed and caught in a paper collection thimble and weighed. The remaining
sample is then subjected to the same conditions for an additional 4-hour
period (attrition phase). Dust and fines generated in the attrition phase
via abrasion, friction, and breakage of the catalyst support are collected
and weighed. The obtained values are used to calculate the attrition index
of the support as follows:
##EQU1##
The apparatus utilized to determine the attrition index is the Roller
apparatus (model no. 5-445) manufactured by the American Instrument
Company of Silver Springs, Maryland. The apparatus is illustrated in FIG.
11 and consists of stainless steel cylindrical tank 11 having an upper
portion and a lower portion that are conically or funnel shaped, all of
which together define a settling chamber 13. The lower conical portion is
a longitudinally extending section that terminates into inlet port 15
which is connected at one end to U-shaped sample tube 17 (1 in. I.D.).
Such inlet connection is a flexible connection permitting vertical
movement of the sample tube for a purpose to be described. The other end
of the sample tube has an inlet port 19 that receives removable jet nozzle
21 to permit the introduction of a given amount of test sample into the
sample tube. The jet nozzle has an orifice (0.07 in.) to provide for the
direction of a high velocity jet of air onto the test sample in the sample
tube. A suitable inlet tube 23 has one end connected to the jet nozzle
while its other end is connected to air supply means that is controlled in
pressure and humidity.
The upper conical portion of tank 11 is a short conical section terminating
into outlet port 25 that is connected via U-shaped collection tube 27 (1
in. I.D.) to paper collection thimble 29 (Whatman paper extraction thimble
123 mm.times.43 mm I.D.). The paper collection thimble permits the flow of
air while trapping any particles contained in the air. The connection
between outlet port 25 and collection tube 27 is a flexible connection to
permit the vibration of tank 11 without interfering with the flow of
particles into the collection thimble.
Suitable rocking actuating means 31 intermittently contact tank 11 and
sample tube 17 to impart a rocking motion to the sample tube (which is in
a vertical direction only) and a moderately vibrating motion to tank 11 to
keep the test sample from clinging to the inside surface of settling
chamber 13. Guide means 33 are connected to the sample tube to restrict
movement of said tube in a vertical direction only.
A weighed 15 ml sample of loosely packed catalyst support is placed into
sample tube 17. Jet nozzle 7 is inserted into inlet port 19 of the sample
tube and a continuous jet of air (50 to 70% relative humidity) is passed
into the sample through the jet nozzle at a flow rate of 21 liters/min.
The sample becomes fluidized in the sample tube and the fines generated
via attrition are forced into settling chamber 13 of tank 11 wherein the
fines and attrited particles are forced up through collection tube 27 and
into collection thimble 29. The attrition index is calculated in
accordance with the method and formula described above.
The invention now being generally described will be better understood by
reference to certain specific examples which are included herein for
purposes of illustration only and are not intended to be limiting of the
invention or any embodiment thereof.
EXAMPLE 1
This example illustrates the preparation of the attrition resistant
alpha-alumina catalyst support material of the present invention.
298 grams of commercially available Ketjen Grade E spheroidally shaped
gamma-alumina particles (80-225 mesh) were calcined in a furnace at
1250.degree. C. for 16 hours to convert the particles to the alpha-alumina
phase. Typical particle properties are given in Table I.
TABLE I
______________________________________
Before After
Properties Calcination
Calcination
______________________________________
Surface Area* (m.sup.2 /g)
151.9 9.3
Total Pore Vol. (ml/g)
.38605 .01862
Ave. Pore Diameter* (A)
101.7 80.3
Alumina Phase Gamma Alpha
Crystal Crack Boundaries
Absent Absent
______________________________________
*B.E.T. Method
EXAMPLE 2
This example compares the attrition resistance of the alpha-alumina
supports of the present invention to conventional alpha-aluminas. Samples
of commercially available Ketjen Grade ES gamma-alumina, Alcoa C-31
alumina trihydrate, and Harshaw (AL-3922 P) gamma-alumina were calcined
for 16 hours at 1050.degree. C. and 1250.degree. C. to obtain the
transitional and alpha phases, respectively. Attrition numbers for each
phase were determined via the Roller attrition test. Results are set forth
in Table II. It is clearly illustrated that the alpha-alumina formed from
the Ketjen gamma-alumina in accordance with the teachings of this
invention has superior attrition resistance compared to the alpha-aluminas
formed from conventional alpha-alumina precursor compounds.
TABLE II
__________________________________________________________________________
Calcination Temp. Surface Area
Starting Material
(.degree.C.)/(Hrs.)
Crystal Phase
Attrition No.
(m.sup.2 /g)
__________________________________________________________________________
Ketjen Grade ES.sup.1
-- gamma 2.7 151.9
gamma-alumina
Ketjen Grade ES.sup.1
1050/(16)
Transition
5.4 20.0
gamma-alumina
Ketjen Grade ES.sup.1
1250/(16)
alpha 7.9 9.3
gamma-alumina
Harshaw AL-3922 P.sup.2
-- gamma 17.5 134.9
Harshaw AL-3922 P.sup.2
1050/(16)
Transition
23.8 20.0
Harshaw AL-3922 P.sup.2
1250/(16)
alpha 72.5* 3.1
Alcoa C-31.sup.3
-- -- 49.8 161.8
(alumina hydrate)
Alcoa C-31.sup.3
1050/(16)
Transition
62.9 13.4
(alumina hydrate)
Alcoa C-31.sup.3
1250/(16)
alpha 90.0* 4.8
(alumina hydrate)
__________________________________________________________________________
.sup.1 available from Ketjen (Subsidiary of Akzo Chemical BV), Amersfoort
Netherlands
.sup.2 available from HarshawFiltrol Chemical Company, Cleveland, OH
.sup.3 available from Aluminum Company of America, Pittsburgh, PA
*Lowest possible value due to a large amount of fines carryover
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